Well, I'm afraid text entry in Web-Decker is a little bit of a mixed bag; it depends on the combination of your operating system, web browser, and your active keyboard layout.
On MacOS, for example, the US-English keyboard layout allows characters like "ñ" to be typed with a "dead-key" combination: option+n to latch in eñe-mode, and then separately "n" to complete the sequence. Problem is, web browsers don't emit any key events for the dead key: Web-Decker only sees "n". If you're using a keyboard layout where ñ can be typed directly- without dead-keys- it will probably work, but it also may not?
On touch devices, Web-Decker will show a touch keyboard, and all the non-ASCII DeckRoman characters can be found under the "alt" key on that keyboard, using a layout loosely adapted from classic MacOS. In unlocked decks, Decker also allows a user to manually summon this keyboard with Edit -> Keycaps... from the menu. The intention is to help people work around limitations of their local keyboard layouts when they need an unusual character, but it only helps for authoring; the menu is hidden in a locked deck.
Copying and pasting text in from another application should always work, as should opening and viewing decks that already contain DeckRoman characters.
Depending on what you're trying to do, some of these workarounds may help, I hope?